Broad acceptance and extensive usage of mobile broadband services put enormous pressure on available cellular network radio resources. Expensive auctioned spectrum must be used efficiently. Vertical cell splitting is a way to increase the reuse pattern of the rare frequency spectrum. The BTS splits one cell into inner and outer cell. The BTS uses two narrow vertical antenna beams, one covering the inner and the other one the outer cell. This increases the maximum system capacity in spectrum limited network. Using active antenna array (AAA), there is a virtual split of the radio resources being available in the outer and inner cell. For example, the available total radio frequency (RF) output power of AAA can be flexibly split for serving the inner and outer cell. As the number of user equipments (UEs) or the throughput demand is changing between the inner and outer cells, the BTS can adjust the allocation via AAA accordingly.
FIGS. 1a and 1b show the scenarios 100a, 100b of a BTS 101 equipped with an AAA serving a cell area. To increase the network capacity, this cell area is split into inner cell 102 and outer cell 104. The inner cell 102 and outer cell 104 are served by separated beams 112, 114, which are formed by the AAA. As the traffic share between the inner cell 102 and outer cell 104 is changing depending on traffic and number of users, the AAA is adaptively adjusting the split of its available radio resources assigned to each of the inner cell 102 and outer cell 104. The radio resources are for example the BTS RF output power. The resource split ratio between the inner cell 102 and outer cell 104 is controlled by BTS 101 measuring the combination of traffic throughput and transmit power demand in each of the inner cell 102 and outer cell 104. FIGS. 1a and 1b show two extreme examples of BTS power distribution for all UEs 103, 105, 107 either in outer cell 104, according to FIG. 1a, or in inner cell 102, according to FIG. 1b. The cell split can be either vertical (as shown in FIGS. 1a and 1b: inner and outer cell) or horizontal (as shown in FIG. 2: left and right cell) depending on AAA used.
FIG. 2 shows the horizontal cell split scenario 200 of a BTS 201 equipped with an AAA serving a cell area. To increase the network capacity, this cell area is split into left cell 202 and right cell 204. The left cell 202 and right cell 204 are served by separated beams 212, 214, which are formed by the AAA. FIG. 2 shows an example of BTS power distribution where a first part of the UEs 203, 205 are located in the left cell 202 and a second part of the users 207, 209 are located in the right cell 204.
For both cell split scenarios as shown in FIGS. 1a, 1b and 2, the legacy implementation of vertical or horizontal cell split implements a fixed allocation of available radio resources. For example, two passive antennas with different tilts are served via two separated radio transceivers. The RF output of these two transceivers is fixed with respect to the inner and outer cells.
As the radio modules and antennas are separated, there is no possibility for a neighboring cell (neighboring radio module and antenna) to “borrow” its unused resources. A low-loaded or no-load cell cannot give its unused resources to another collocated radio cell.
As an example, two cells are created by collocated two passive antennas and two radio transceivers. Both transceivers are dimensioned to deliver 20 W RF output power each, in order to serve the maximum cell capacity. In the case of one of the two cells being overloaded, limited by available RF power, this overloaded cell cannot “borrow” the RF output power from the collocated second cell.